In acousto-optical imaging, one detects the beat of a local oscillator with an acoustic component of a signal wave scattered by an object to be imaged, shifted in frequency by the vibration at an acoustic frequency of a point of said object to be imaged from which one seeks to obtain information of an optical nature.
In monopixel acousto-optical imaging, the measurement of the beat between the component of the signal wave scattered without frequency shift, which serves as local oscillator, and the acoustic component of the signal wave, which carries the information, exhibits significant noise since the measurement of said beat is performed at just one point of the detection plane whereas each of these two components varies in a random manner in this plane. It is necessary to perform a summation over time of the square of the amplitude of said beat to obtain information exhibiting a better signal/noise ratio.
To alleviate this drawback, it is possible to use a multipixel detection device, by performing a summation over the pixels of the detection device, rather than in time, such as described in “Ultrasonic tagging of photon paths in scattering media: parallel speckle modulation processing”, Optics Letters, Vol. 24, No. 3, 1st Feb. 1999, page 181. In this context, it is necessary to modulate the power of the laser at a frequency close to the acoustic frequency, so that the beat between the acoustic component of the signal wave, carrying the information, and the modulation lateral band (the local oscillator) is of sufficiently low frequency to be detected by a multipixel detection device, which in general possesses a low acquisition frequency. Nevertheless, a major problem remains, in that the weight of the local oscillator is in general too low. The heterodyne gain is then too low to be able to perform heterodyne detection with optimal noise.
The present invention is aimed in particular at alleviating these drawbacks.